The switching of the base station is illustrated in FIG. 1 which shows the main components of a cellular network constructed in a way known per se. The system includes at least one center MSC (Mobile Services Switching Center) which is connected to a public telephone network NET, for instance. The center communicates with controllers BSC 1-3 (Base Station Controller) of the base station which controllers may comprise one or more base stations BTS (Base Transceiver Station). In addition, several mobile stations (MS) travel in the network in the zones (not shown) of the cells of the base stations BTS. For the sake of clarity only one mobile station, a mobile telephone in this instance, is illustrated. Intercommunications within the network can be established between mobile stations MS and/or between mobile station MS and network NET.
When travelling in the area of the network, mobile station MS can reach the border of the cell, i.e., coverage area of its current base station BTS, and a link has to be established with another base station BTS which is closer so that the mobile station can move from one cell of the network to another without an interruption occurring in the intercommunication in progress. In practice this means that when the base station is switched, the channel used between the mobile station and the base station is switched. In the TDMA system the channel means the time slot in which the procedures are carried out. (Switching the channel may also be required when moving within one cell.) The term "handover" is also used when speaking of channel switching and in FIG. 1 it is illustrated in curves a (switch 1).fwdarw.2)) and b (switch 2).fwdarw.3)).
In order to ensure an interference-free handover the mobile station monitors at regular intervals the quality of reception, for instance, the strength of the signal of its own base station and that of the next nearest base stations and sends the results to the base station BTS it uses at that moment, from where the data is directed to controller BSC and if so required, to center MSC. On the basis of the monitoring results the center, and in the future perhaps the mobile station as well, can specify a new base station for the mobile station and the exact moment for the handover, when, for example, the communication with the old base station has diminished.
FIG. 2 illustrates a situation where the mobile station MS is transferred from cell 1 of base station BTS1 to cell 2 of another base station BTS2. The transfer is illustrated in arrow A. Mobile station MS first communicates, on radio link R1, with base station BTS1, which monitors the power of the signal of MS during the communication and reports on it to center MSC, when a handover to base station BTS2 under base station controller BSC2 is expected to occur. Mobile station MS monitors once in a while (dining the monitoring cycle), based on the neighbouring base station list it has received, the signal of its neighbouring base stations as well, in this case the signal of base station BTS2, and reports on it to BTS1. When the handover criterion is met, a message is signalled to base station controller BSC2 including the parameters required in recognizing mobile station MS and the data of the new channel (time slot) which will be used in communication between mobile station MS and base station BTS2. After preparation the handover to the new base station is performed, controlled by MSC. The signalling required in this operation is illustrated in mows S1 to S3.
A perfect handover should occur without the user noticing any interferences. For this purpose, different methods for handover have been developed and researched in different cellular systems.
A soft handover is considered an advantage offered by the CDMA system. In the CDMA system (Code Division Multiple Access), several base stations are capable of receiving a signal transmitted by a mobile station by using the same code. Similarly, several base stations may send the same signal to a mobile station using the same code, and the moving station receives the signals like signals coming through multipath propagation. The CDMA system is being developed for a digital mobile station network in the U.S.A., for instance. Another significant mobile station network system is based on the TDMA (Time Division Multiple Access) which is used or will be used, for instance, in the GSM system (Global System for Mobile Communication), the DECT system (Digital European Cordless Telephone) for cordless telephones, and the digital (ANSI) mobile station system in the U.S.A.
The soft handover described above with respect to the CDMA system could also be implemented in the TDMA system. Thus two or more base stations can receive a signal transmitted by a mobile station simultaneously. This is often referred to as macrodiversity. However, this requires that the base stations are either synchronized to each other (because otherwise the base stations cannot receive at the same time but the transmission bursts overlap in the other base station) or that the other base station uses a separate carrier and a receiver for reception. This type of arrangements make the network more complicated and its use more difficult.
In the above-mentioned cases, problems can also occur in the TDMA network regarding the exact adjustment of the time alignment of the mobile station because mobile station MS adjusts its transmission in accordance with one base station, but the same time alignment does not necessarily suit the other base station. On the other hand, the extended guard time between bursts could be used but this would lead to an unnecessarily large requirement for capacity.
In the TDMA system, it is possible to arrange so that two or more base stations send the same signal to the mobile station "simultaneously" by using the same time slot and carrier wave frequency. Such an operation is called "simulcasting" (sometimes also macrodiversity). This type of transfer would also require synchronization of the base stations. If the mobile station receives a signal from two base stations simultaneously (and the signals cannot be distinguished from each other within the resolution of the receiver) and if the signals are approximately equally strong, this will create a strong standing wave pattern in which the signals either strengthen or cancel each other, whereby the mobile station experiences this as strong fading. This type of fading can have a greater effect than fading due to multipath propagation. This problem could be solved by using a suitable delay between the base stations, whereby signals would arrive at the mobile station at different points in lime and would not cancel each other but they could be utilized by a Viterbi type of receiver, for instance. However, adjusting the delay would be difficult, at least when the mobile station MS is moving. In addition, adjusting the delay would substantially increase the signalling between the base station and the mobile station. Besides, the adjustment of the delay must stay within a limited range because if the delay between the signals increases too much, the receiver is not able to use both signals. Simulcasting also adds to the interference level of the whole network because a particular frequency is used in adjacent base stations.